HUDs and NEDs comprise tree fundamental parts: a light projector, a computing unit controlling the projector, and an optical combiner which is adapted to show the light from the projector to a see-through display allowing the user of the HUD or NED to see both the scenery behind the display and the projected light without requiring the user to look away from his usual viewpoint. The optical combiner may be based on diffractive optical elements, sometimes referred to as holographic optical elements (HOEs).
WO 2006/064301 discusses near-to-eye display devices comprising diffractive elements for coupling light into an optical substrate and out of the optical substrate. Previous versions, further developments and variations of devices operating with the same general principle are introduced in WO 99/52002, WO 2009/077802, WO 2009/077803 and WO 2011/110728.
US 2009/0245730 discloses a display device operating with the same principle, wherein at least one of the two diffraction gratings is a binary-blazed grating having a multiplicity of diffraction structures, which are composed of a multiplicity of individual substructures that ensure a blaze effect and in plan view have the shape of a closed geometrical surface. The proposed structure aim at providing an optical display device with which the light can both be coupled into the plane plate of the light guide with the highest possible diffraction efficiency by the input grating and coupled out again homogeneously by the output grating.
WO 2011/113662 discloses a diffractive combiner for a color head-up display (HUD) device. The device includes a first optical diffraction grating adapted for diffracting, in a diffraction direction, light having a first wavelength and which is incident on the first grating in an incidence direction, a second optical diffraction grating adapted for diffracting, in the same direction, light having a second wavelength and which is incident on the second grating in the incidence direction. The first and second optical diffraction gratings are formed in relief on first and second opposite surfaces of the combiner. The first and/or second grating is made as a wavelength multiplex optical diffraction grating and is adapted for diffracting in the diffraction direction light at a third wavelength impacting the first and/or second optical diffraction grating in the incidence direction.
At least some of the abovementioned solutions, however, suffer from an undesired effect caused by transmitted light coming through the diffracting gratings to the observer's eye, i.e. the so-called rainbow effect. The rainbow effect shows like a colourful visible pattern in addition to the desired diffracted image. This may make the diffractive combiner element technology practically useless in some applications, unless the problem is solved.
U.S. Pat. No. 4,856,869 discloses a display element including a substrate and a display pattern formed on the substrate, the display pattern having a first diffraction grating structure and a second diffraction grating structure. The direction of the grating lines of the first diffraction grating structure differs from that of the grating lines of the second diffraction grating structure, thereby aiming at preventing the occurrence of a rainbow-like image. The proposed solution is periodic in two directions which causes light to diffract into multiple directions, making individual diffraction orders weaker. Transmitted diffraction orders are, however still considerably strong, whereby the result is far from optimal regarding the rainbow interference image. In addition, the structure is relatively difficult to manufacture.
Thus, there is a need for improved optical devices.